Power circuit

ABSTRACT

A power circuit includes a boost circuit having a boost switching element, a coil provided between a battery and the boost switching element and a rectifier element provided between the coil and a load, a first switch connected to the boost circuit in parallel, a current detection circuit detecting current flowing through the first switch and a control circuit turning on the first switch during a normal state other than a restart of an engine after an idling stop and turning off the first switch and controlling an operation of the boost switching element at a restart of the engine after an idling stop so that voltage of the battery is boosted and supplied to the load. The control circuit determines based on the current detected by the current detection circuit during the normal state whether or not the first switch has an open fault.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Application No. 2012-080848filed Mar. 30, 2012.

BACKGROUND

The present invention relates to a power circuit that allows a batteryto supply voltage stably to a load in a vehicle with an idling stopcontrol.

Recently a vehicle with an idling stop control has been put to practicaluse for reducing fuel consumption and exhaust gas emission. The idlingstop control of the vehicle turns off its engine automatically when itis detected that the vehicle stops e.g. at a traffic light and restartsthe engine automatically when it is detected that the vehicle is aboutto start again.

When the engine of such vehicle is restarted after an idling stop, alarge current flows to a starter motor of the vehicle, so that thevoltage of a battery of the vehicle is dropped temporarily. Accordingly,the voltage of power to various loads such as electronic devices otherthan the starter motor which are connected to the battery is alsodropped temporarily at a restart of the vehicle. Therefore, there is afear that the voltage falls below a level that is required for anyelectronic device to operate properly, thus inviting a temporarymalfunction of the device. For example, a car navigation system and anaudio system may be reset or a sound may jump in the audio system. Thus,unforeseeable operation of any electronic device may occur.

To solve the above problem, a power circuit may be provided between thebattery and the load so as to keep constant the voltage required for theloads even when a voltage drop of the battery occurs temporarily.

Japanese Patent Application Publication 2005-112250 discloses a powercircuit to solve the above problem. The power circuit includes a boostcircuit and a bypass relay that bypasses the boost circuit when therelay is closed. At a restart of an engine after an idling stop, theboost circuit is activated with the bypass relay kept opened, so thatthe voltage of a battery is boosted by the boost circuit and the boostedvoltage is supplied to various electronic loads of the vehicle. During anormal state other than the restart of the engine, the bypass relay iskept closed thereby to bypass the boost circuit, so that the voltage ofthe battery is supplied to the loads through the bypass relay.

When the voltage of the battery is dropped temporarily at the restart ofthe engine after an idling stop, the power circuit ensures that thevoltage required for the loads is supplied. Meanwhile, during the normalstate other than the restart of the engine, the voltage of the batteryis supplied to the loads without being dropped by a switching element inthe boost circuit, so that supply of the voltage required for the loadscan be maintained.

However, if an open fault occurs in the bypass relay in the powercircuit and the voltage of the battery is supplied at all times to theloads through the boost circuit, the voltage of the battery to besupplied to the loads is dropped by the switching element in the boostcircuit.

The present invention is directed to providing a power circuit thatallows supply of a stable voltage to loads of electronic devices of avehicle equipped with an idling stop control.

SUMMARY

A power circuit includes a boost circuit having a boost switchingelement, a coil provided between a battery and the boost switchingelement and a rectifier element provided between the coil and a load, afirst switch connected to the boost circuit in parallel, a currentdetection circuit detecting current flowing through the first switch anda control circuit turning on the first switch during a normal stateother than a restart of an engine after an idling stop and turning offthe first switch and controlling an operation of the boost switchingelement at a restart of the engine after an idling stop so that voltageof the battery is boosted and supplied to the load. The control circuitdetermines based on the current detected by the current detectioncircuit during the normal state whether or not the first switch has anopen fault.

Other aspects and advantages of the invention will become apparent fromthe following description, taken in conjunction with the accompanyingdrawings, illustrating by way of example the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention that are believed to be novel areset forth with particularity in the appended claims. The inventiontogether with objects and advantages thereof, may best be understood byreference to the following description of the presently preferredembodiments together with the accompanying drawings in which:

FIG. 1 is a schematic configuration diagram showing a power circuitaccording to a first embodiment of the present invention;

FIG. 2 is a flowchart showing the operation of the power circuit of FIG.1;

FIG. 3 is a schematic configuration diagram showing a power circuitaccording to a second embodiment of the present invention;

FIG. 4 is a schematic configuration diagram showing a power circuitaccording to a third embodiment of the present invention;

FIG. 5 is a schematic configuration diagram showing a power circuitaccording to a fourth embodiment of the present invention;

FIG. 6 is a schematic configuration diagram showing a power circuitaccording to a fifth embodiment of the present invention;

FIG. 7 is a schematic configuration diagram showing a power circuitaccording to a sixth embodiment of the present invention; and

FIG. 8 is a schematic configuration diagram showing a power circuitaccording to a seventh embodiment of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Referring to FIG. 1, the power circuit according to the first embodimentof the present invention is generally designated by numeral 1. The powercircuit 1 is adapted to keep constant the voltage of a battery 2 whichis mounted on a no-idling vehicle and also to supply the voltage to aload 3. The power circuit 1 includes a boost circuit 4 and a bypasscircuit 5.

The boost circuit 4 boosts the voltage of the battery 2 and supplies theboosted voltage to the load 3 at a restart of an engine after an idlingstop of the engine. The boost circuit 4 includes a boost switchingelement 6, a coil 7, a rectifier diode 8 (or a rectifier element),capacitors 9, 10, a drive circuit 11, a control circuit 13 and a powersource 12 for the control circuit 13.

The boost switching element 6 is e.g. of Metal Oxide Semiconductor FieldEffect Transistor (MOSFET) or Insulated Gate Bipolar Transistor (IGBT).

The coil 7 is provided between the battery 2 and the boost switchingelement 6. The rectifier diode 8 is provided between the coil 7 and theload 3. The capacitor 9 is provided in the input stage of the boostcircuit 4.

The capacitor 10 is provided in the output stage of the boost circuit 4.The drive circuit 11 drives the boost switching element 6 based on acontrol signal S1 transmitted by the control circuit 13.

The power source 12 supplies power to the control circuit 13. The bypasscircuit 5 includes a first switch 14, a second switch 15, drive circuits16, 17 and a current detection circuit 18.

The first and the second switches 14, 15 are e.g. of a MOSFET or a relayand connected to the boost circuit 4 in parallel. The drive circuit 16drives the first switch 14 based on a control signal S2 transmitted bythe control circuit 13 and the drive circuit 17 drives the second switch15 based on a control signal S3 transmitted by the control circuit 13.

The current detection circuit 18 is made e.g. of a shunt resistance, acurrent transformer or a Hall element and serves as a current detectionsensor. The current detection circuit 18 is provided between theconnecting point of the first and the second switches 14, 15 and theload 3.

The control circuit 13 transmits control signals S1 through S3 based onvarious data sent by a head control circuit 19. The control circuit 13consists of software or hardware. The control circuit 13 consisting ofsoftware includes a CPU and a memory. A program stored in the memory isread and executed by the CPU. The control circuit 13 may be providedoutside the boost circuit 4.

During the normal state other than the restart of the engine subsequentto an idling stop, the control circuit 13 transmits control signals S2,S3 for keeping the first and the second switches 14, 15 on or closedbased on various data sent from the head control circuit 19 and also acontrol signal S1 for keeping the boost switching element 6 off oropened. During such normal state, the battery 2 is electricallyconnected to the load 3 through the then closed first and secondswitches 14, 15, so that electric current flows from the battery 2 tothe load 3 through the switches 14, 15, bypassing the boost circuit 4.Therefore, no boosting of the voltage of the battery 2 is performed bythe boost circuit 4. Supplying of the required voltage to the load 3during the normal state other than restarting of the engine subsequentto an idling stop can be maintained by making the voltage drop of thebattery 2 due to the first and the second switches 14, 15 of the bypasscircuit 5 smaller than that due to the coil 7 and the rectifier diode 8of the boost circuit 4. This mode of operation of the control circuit 13will be hereinafter referred to as “bypass mode”.

At a restart of the engine after an idling stop, the control circuit 13transmits the control signals S2, S3 as well as S1, wherein the signalsS2, S3 are used for keeping the first and the second switches 14, 15 offor opened and the signal S1 for causing the boost switching element 6 torepeat on and off operation. Thus, the electrical connection between thebattery 2 and the load 3 through the first and the second switches 14,15 is cut off, but the boost circuit 4 is activated to boost the voltageof the battery 2, so that the boosted voltage of the battery 2 issupplied to the load 3. Therefore, even if the voltage of the battery 2is dropped temporarily due to the operation of the starter motor forrestarting of the engine, supplying of the voltage required for the load3 can be maintained. This mode of operation of the control circuit 13will be hereinafter referred to as “boost mode”.

In the bypass mode operation, the control circuit 13 determines whetheror not the current detected by the current detection circuit 18 at apredetermined time interval (e.g. 5 seconds) is at a threshold value lth(e.g. 5 A (amperes)) or higher. If the control circuit 13 determinesthat the detected current is less than lth, in other words, when theload 3 is not driven and no current (e.g. 5 A) is flowed to the load 3,the bypass mode is continued. On the other hand, if the control circuit13 determines that the detected current is at lth or more, in otherwords, when the load 3 is being driven and a current is flowed to theload 3, it will be determined whether or not either or both of the firstand the second switches 14, 15 suffers from open fault.

In the bypass mode of operation, the control circuit 13 determineswhether or not the first and the second switches 14, 15 have an openfault in the following manner. The control circuit 13 transmits thecontrol signals S2, S3 to keep the first switch 14 on or closed and keepthe second switch 15 off or opened for a predetermined length of time(e.g. 5 mS), respectively. If the current detected by the currentdetection circuit 18 is 0 A or substantially 0 A, the control circuit 13determines that first switch 14 has an open fault and informs the headcontrol circuit 19 (or output means) of the result. Subsequently, thehead control circuit 19 gives the warning that the first switch 14 hasan open fault on a display 20 (or output means) or by a speaker 20 (oroutput means).

The control circuit 13 also transmits the control signals S2, S3 to keepthe first switch 14 off or opened and keep the second switch 15 on orclosed for a predetermined length of time (e.g. 5 mS), respectively. Ifthe current detected by the current detection circuit 18 is 0 A orsubstantially 0 A, the control circuit 13 determines that the secondswitch 15 has an open fault and informs the head control circuit 19 (oroutput means) of the result. The head control circuit 19 gives thewarning that the second switch 15 has an open fault on the display 20 orby the speaker 21. Thus, during the normal state, the control circuit 13turns off the first switch 14 and the second switch 15 alternately at aregular time interval (e.g. 5 S) and determines based on the currentdetected by the current detection circuit 18 whether or not the firstswitch 14 has an open fault while the second switch 15 is turned off andalso whether or not the second switch 15 has an open fault while thefirst switch 14 is turned off.

FIG. 2 is the flowchart showing the operation of the control circuit 13of FIG. 1. It is assumed that the first and the second switches 14, 15are both turned on or closed in the initial state (e.g. in a parkedstate before starting the engine).

If the control circuit 13 is informed by the head control circuit 19that an ignition signal (IG) is turned to a high level by user's (e.g.driver's) manipulation of the ignition switch (if Yes at S21), the boostmode operation will be executed until a predetermined length of time haselapsed (S22-S24). It is assumed that the starter motor is started todrive the engine of the vehicle when the ignition signal is turned to ahigh level and also that the predetermined length of time issubstantially the same as the length of time of the voltage drop of thebattery 2 due to the operation of the starter motor.

The control circuit 13 reads the current detected by the currentdetection circuit 18 and sets the read current as the offset value(S25). In other words, the control circuit 13 sets the value of thecurrent detected by the current detection circuit 18 as the offset valueduring the time after the end of the boost mode operation and alsobefore the start of the bypass mode operation (or the time when thefirst and the second switches 14, 15 are both kept off or opened andalso the boost switching element 6 is kept off or opened).

After a start of the bypass mode operation (S26), if the control circuit13 is informed by the head control circuit 19 of a restart of the engineafter an idling stop (if Yes at S27), the process returns to S22.

On the other hand, if the control circuit 13 is not informed by the headcontrol circuit 19 of a restart of the engine after an idling stop (ifNo at S27) and also the predetermined length of time (5 seconds) haselapsed (if Yes at S28) after the start of the bypass mode (S26), thecontrol circuit 13 reads the current detected by the current detectioncircuit 18 (S29) and determines whether or not the value of the currentis at the threshold value lth or more (S30).

If the control circuit 13 determines that the value of the current thatis read at S29 is less than the threshold value lzh (if No at S30) andsubsequently the control circuit 13 is informed by the head controlcircuit 19 of a restart of the engine after an idling stop (if Yes atS31), the control circuit 13 finishes the bypass mode operation (S32)and returns the process to S22.

On the other hand, if the control circuit 13 determines that the valueof the current read at S29 is less than the threshold value lzh (if Noat S30) and subsequently the control circuit 13 receives no informationfrom the head control circuit 19 of a restart of the engine after anidling stop (if No at S31), the control circuit 13 reads again thecurrent detected by the current detection circuit 18 (S29) after anelapse of the predetermined length of time (Yes at S28) and determineswhether or not the value of the current is at the threshold value lth ormore (S30).

When the control circuit 13 determines that the value of the currentread at S29 is at the threshold value lzh or more (if Yes at S30), thecontrol circuit 13 turns on the first switch 14 and turns off the secondswitch 15 (S33). Subsequently, the control circuit 13 reads the currentdetected by the current detection circuit 18 (S34) and corrects thecurrent by the offset value that has been set at S25 and determineswhether or not the corrected current is 0 A or substantially 0 A (S35).Specifically, the control circuit 13 sets the value that is figured outby subtracting the offset value set at S25 from the value of the currentdetected at S34 as the corrected current value at S35.

If the control circuit 13 determines that the corrected current value is0 A or substantially 0 A (if Yes at S35), the control circuit 13 informsthe head control circuit 19 that the first switch 14 has an open fault(S36) and the process goes to S37.

On the other hand, when the control circuit 13 determines that thecorrected current value is neither 0 A nor substantially 0 A (S35 isNo), the control circuit 13 turns off the first switch 14 and turns onthe second switch 15 (S17). Subsequently, the control circuit 13 readsthe current detected by the current detection circuit 18 (S38) andcorrects the current by the offset value set at S25 and determineswhether or not the corrected current value is 0 A or substantially 0 A(S39). Specifically, the control circuit 13 sets the value that isfigured out by subtracting the offset value set at S25 from the value ofthe current detected at S38 as the corrected current value at S39.

If the control circuit 13 determines that the corrected current value is0 A or substantially 0 A (if Yes at S39), the control circuit 13 informsthe head control circuit 19 that the second switch 15 has an open fault(S40) and the process goes to S41.

On the other hand, if the control circuit 13 determines that thecorrected current value is neither 0 A nor substantially 0 A (S39 isNo), the control circuit 13 turns on the first and the second switches14, 15 and determines whether or not the control circuit 13 per se isinformed by the head control circuit 19 of a restart of the engine afteran idling stop (S31).

According to the power circuit 1 of the first embodiment having thefirst and the second switches 14, 15, even if either one of the firstand the second switches becomes unserviceable due to an open fault, thevoltage of the battery 2 can be maintained and supplied to the load 3 ofthe vehicle through the other usable switch during the normal stateother than a restart of the engine after an idling stop. Thus, thevoltage of the battery 2 can be supplied stably to the load 3 of avehicle with an idling stop control. Additionally, the provision of thetwo switches 14, 15 in the bypass circuit 5 permits the battery 2 tosupply its voltage to the load 3 through one of the switches 14, 15 evenif the other of the switches 14, 15 is faulty. As a result, no currentflows from the battery 2 to the boost circuit 4 during the bypass modeoperation and, therefore, heat generation of the coil 7 or the rectifierdiode 8 is prevented, with the result that the coil 7 or the rectifierdiode 8 can be protected against damage due to such heat.

The electronic devices that are related to fundamental performances of avehicle such as traveling, turning and stopping and, therefore, need besupplied with a strictly controlled voltage can be used as the load 3connected to the power circuit 1 that supplies power stably.Additionally, the power circuit 1 is configured so that no current flowsfrom the battery 2 to the load 3 through the boost circuit 4 during thenormal state operation of the engine. Therefore, the boost circuit 4 ofthe power circuit 1 need not use a large-capacity element for the coil 7and the rectifier diode 8, which helps to decrease the cost of the powercircuit 1.

The power circuit 1 according to the first embodiment which determineswhether or not either or both of the first and the second switches 14,15 has an open fault and informs the vehicle user of the state of theswitches 14, 15 by the display 20 or the speaker 21 allows the user torecognize any trouble of the first switch 14 or the second switch 15.Therefore, the user can be prompted to replace the faulty switch or aunit including the faulty switch with a new one so as to restore thepower circuit 1 timely.

The power circuit 1 shown in FIG. 1 has only one current detectioncircuit 18, but may have two or more current detection circuits 18. Forexample, the power circuit 1 according to the second embodiment includestwo current detection circuits, as shown in FIG. 3. The bypass circuit 5of the power circuit 1 has a current detection circuit 31 for detectingthe current flowing through the first switch 14 and a current detectioncircuit 32 for detecting the current flowing through the second switch15. In the control circuit 13 shown in FIG. 3, during the time after theboost mode operation and also before the bypass mode operation (or whenthe first and the second switches 14, 15 are both turned off and theboost switching element 6 is also turned off), the values of thecurrents then flowing through the first and the second switches 14, 15are set as the offset values A and B, respectively. In the controlcircuit 13 shown in FIG. 3, if the sum of the values of the currentsdetected by the current detection circuits 31, 32 is at the thresholdvalue lth or more during the bypass mode operation and it is determinedthat at least one of the values of the currents read by the currentdetection circuits 31, 32 is 0 A or substantially 0 A, the head controlcircuit 19 is informed that the switch corresponding to the currentdetection circuit that shows 0 A or substantially 0 A has an open fault.According to the second embodiment, in determining the open fault of thefirst switch 14 or the second switch 15, the first and the secondswitches 14, 15 need not be turned off alternately and also the currentsneed not be read many times, so that the operation of the controlcircuit 13 can be simplified. Additionally, the current detectioncircuits are redundant in the power circuit 1 shown in FIG. 3, so thatthe accuracy of determining the open fault of the first switch 14 or thesecond switch 15 can be improved.

The power circuit 1 shown in FIG. 1 or 3 is configured so that the openfault of the first switch 14 or the second switch 15 is determined basedon the current detected by the current detection circuits 18, 31, 32.The power circuit 1 according to the third embodiment shown in FIG. 4differs from the power circuits 1 of the first and the secondembodiments in that a potential difference detection circuit 41 isprovided which detects the potential difference between the input andthe output voltages of the boost circuit 4 and transmits a signal thatis indicative of the detected potential difference to the controlcircuit 13 which determines whether or not an open fault is present inany of the first switch 14 and the second switch 15 based on thedetection signal from the potential difference detection circuit 41. Ifeither one of the first and the second switches 14, 15 has an opendefault when one of the switches 14, 15 is turned on and the other ofthe switches 14, 15 is turned off, the current flows from the battery 2to the load 3 through the boost circuit 4 without flowing through thebypass circuit 5. The voltage drop of the battery 2 due to the coil 7and the rectifier diode 8 in the boost circuit 4 is greater than thatdue to the first and the second switches 14, 15 in the bypass circuit 5.Therefore, the potential difference detected by the potential differencedetection circuit 41 when the current flows from the battery 2 to theload 3 through the boost circuit 4 is greater than in the case when thecurrent flows from the battery 2 to the load 3 through the bypasscircuit 5. During the bypass mode operation, if the potential differenceoutputted by the potential difference detection circuit 41 is at thethreshold value Vth or greater when one of the first and the secondswitches 14, 15 is turned on and the other of the switches 14, 15 isturned off, the control circuit 13 of FIG. 4 determines that one of thefirst and the second switches 14, 15 has an open fault and informs thehead control circuit 19 of the result. The threshold value Vth may be orsubstantially the same as the potential difference outputted by thepotential difference detection circuit 41 when a current flows from thebattery 2 to the load 3 through the bypass circuit 5 at the bypass mode.

In the power circuit 1 shown in FIG. 1, 3 or 4, the rectifier diode 8serves as the rectifier element in the respective boost circuits 4. Thepower circuit 1 according to the fourth embodiment shown in FIG. 5differs from the foregoing embodiments of FIGS. 1, 3 and 4 in that arectifier switching element 51 (e.g. IGBT having MOSFET and diodeconnected in parallel) is used to serve as the rectifier element in theboost circuit 4. A drive circuit 52 is provided in the boost circuit 4to drive the rectifier switching element 51 based on the control signalS4 transmitted by the control circuit 13. The control circuit 13 shownin FIG. 5 turns on and off the boost switching element 6 and therectifier switching element 51 alternately during the boost modeoperation. The power circuit 1 of this embodiment can reduce the energyloss due to the rectifier element as compared with a case wherein therectifier diode 8 serves as the rectifier element in the boost circuit4.

The power circuit 1 shown in FIG. 1, 3, 4 or 5 is so configured that thefirst and the second switches 14, 15 in the bypass circuit 5 areconnected to the boost circuit 4 in parallel. However, the bypasscircuit 5 may have more than two switches connected to the boost circuit4 in parallel. For example, the power circuit 1 according to the fifthembodiment shown in FIG. 6 has three switches 14, 15 and 61 which areconnected in parallel to the boost circuit 4. As shown FIG. 6, a drivecircuit 62 is provided in the bypass circuit 5 to drive the switch 61based on the control signal S5 transmitted by the control circuit 13.During the bypass mode operation, if the current detected by the currentdetection circuit 18 is 0 A or substantially 0 A when the controlcircuit 13 turns off the first and the second switches 14, 15 and turnson the switch 61, it is determined that the switch 61 has an open faultand the head control circuit 19 (or output means) is informed by thecontrol circuit 13 that the switch 61 has an open fault. When the headcontrol circuit 19 is so informed, the head control circuit 19 gives amessage informing that the switch 61 has an open fault on the display 20(or output means) or by the speaker 21 (or output means). Thusincreasing the number of switches in the bypass circuit 5, theprobability of a state wherein all the switches in the bypass circuit 5are disabled can be reduced, so that the stability of supplying thevoltage of the battery 2 to the load 3 can be improved.

The power circuit 1 shown in FIG. 1, 3, 4 or 5 is so configured thateach of the first and the second switches 14, 15 is connected to theboost circuit 4 in parallel. However, the power circuit 1 may beconfigured so that only one switch in the bypass circuit 5 is connectedto the boost circuit 4 in parallel. Such examples are shown in FIGS. 7and 8 illustrating the power circuit 1 according to the sixth and theseventh embodiments, wherein only one switch designated by 14 isconnected to the boost circuit 4 in parallel, respectively.

What is claimed:
 1. A power circuit comprising: a boost circuit, whereinthe boost circuit includes: a boost switching element; a coil providedbetween a battery and the boost switching element; and a rectifierelement provided between the coil and a load, a first switch connectedto the boost circuit in parallel; a current detection circuit detectingcurrent flowing through the first switch; and a control circuit turningon the first switch during a normal state other than a restart of anengine after an idling stop, the control circuit turning off the firstswitch and controlling an operation of the boost switching element at arestart of the engine after an idling stop so that voltage of thebattery is boosted and supplied to the load, wherein the control circuitdetermines based on the current detected by the current detectioncircuit during the normal state whether or not the first switch has anopen fault.
 2. The power circuit according to claim 1, furthercomprising: a second switch connected to the boost circuit in parallel,wherein the current detection circuit detects current flowing throughthe second switch, wherein during the normal state, the control circuitturns off the first switch and the second switch alternately at aregular time interval and determines based on the current detected bythe current detection circuit whether or not the second switch has anopen fault while the first switch is turned off and also whether or notthe first switch has an open fault while the second switch is turnedoff.
 3. The power circuit according to claim 1, further comprising: asecond switch connected to the boost circuit in parallel; and anothercurrent detection circuit detecting current flowing through the secondswitch, wherein the control circuit determines based on the currentsdetected by the respective current detection circuits during the normalstate whether or not either or both of the first switch and the secondswitch has an open fault.
 4. The power circuit according to claim 1,wherein a rectifier switching element serves as the rectifier element.5. A power circuit comprising: a boost circuit, wherein the boostcircuit includes: a boost switching element; a coil provided between abattery and the boost switching element; and a rectifier elementprovided between the coil and a load, a first switch connected to theboost circuit in parallel; a potential difference output circuitdetecting potential difference between input voltage and output voltageof the boost circuit; and a control circuit turning on the first switchduring a normal state other than a restart of an engine after an idlingstop, the control circuit turning off the first switch and controllingan operation of the boost switching element at a restart of the engineafter an idling stop so that voltage of the battery is boosted andsupplied to the load, wherein the control circuit determines based onthe potential difference detected by the potential difference outputcircuit during the normal state whether or not the first switch has anopen fault.